A series HEV (Hybrid Electrical Vehicle) includes a motor, a generator and an internal combustion engine and is driven by means of a driving force of the motor. The internal combustion engine is used to generate electrical power. The electrical power generated in the generator by means of the power of the internal combustion engine is used to charge a battery or is supplied to the motor. In the following description, the internal combustion engine and the generator 111 will collectively be referred to as an “auxiliary power unit (APU) 121.
The series HEV executes an “EV driving” or a “series driving.” In an EV driving mode, the HEV is driven by means of a driving force of the motor which is driven by means of electric power which is supplied from a battery. As this occurs, the internal combustion engine is not driven. In a series driving mode, the HEV is driven by means of a driving force of the motor which is driven by means of electric power supplied from both the battery and the APU or electric power supplied only from the APU. As this occurs, the internal combustion engine is driven to drive the generator to generate electric power.
In a plug-in HEV (PHEV), a driving using a CD (Charge Depleting) mode or a driving using a CS (Charge Sustaining) mode is executed. In the CD mode, a motor is driven only by electric power supplied from a battery which is mainly charged by an external or off board commercial power supply, and the SOC (State of Charge) of the battery is lowered according to driving of the motor or the like. In the CS mode, generated electric power obtained from the generator which is mainly driven by an internal combustion engine is used to drive the motor auditor to charge the battery, and the SOC of the battery is maintained substantially constant. Even in the CD mode, the internal combustion engine can be driven according to a required output which is derived from a vehicle speed and an accelerator pedal position and the SOC of the battery. Even in the CS mode, the driving of the internal combustion engine can be stopped according to a required output and the SOC of the battery.
FIG. 19 shows graphs showing an example of changes with time of consumed energy, SOC of the battery, generated electric power by the generator and vehicle speed as the PHEV is driven. In the graph of the consumed energy in FIG. 19, bars which are not hatched indicate energy based on electric power supplied from the battery, and bars which are hatched indicate energy based on electric power supplied from the APU. Portions of the consumed energy indicated by negative values indicate a portion of the power supplied power from APU, which is charged to the battery.
In the example shown in FIG. 19, the CD mode is selected when the PHEV starts from rest, and the PHEV is driven by driving the motor only by electric power supplied from the battery which is charged to a sufficient level. Thereafter, energy is consumed according to the driving of the PHEV, and the SOC of the battery is lowered. The driving mode of the PHEV is switched to the CS mode when the SOC of the battery is lowered to a threshold th, and the internal combustion engine is started. Thereafter, although the SOC of the battery is lowered below the threshold the as a result of the energy being consumed in the motor by the driving of the PHEV, the battery can be charged by generated electric power obtained from the generator which is driven by the internal combustion engine. As a result, the SOC of the battery is maintained substantially constant. In this way, the CD mode is selected mainly when the SOC of the battery is higher than the threshold th, and the CS mode is selected mainly when the SOC of the battery is lower than the threshold.